1. Field of the Invention
The present invention relates to the repair of a defective pixel in an organic electroluminescence (referred to simply as “EL” hereinafter) panel.
2. Description of the Related Art
Conventionally, organic EL display panels have been known as one type of flat display panel. Unlike liquid crystal display (LCD) panels, the organic EL display panels are self-emissive and commercialization of the organic EL display panels as clear and bright displays are highly expected.
An organic EL display is constructed by arranging a plurality of pixels each having an organic EL element in a matrix form. As a method for driving the organic EL element, similar as in LCDs, a passive method and an active method are known. Of these, also similar to the case with LCDs, the active matrix method is more desirable. That is, an active matrix method in which a switching element is provided for each pixel and the display in each pixel is controlled by controlling the switching element is more advantageous than a passive method which does not have a switching element in each pixel, because the active matrix method allows for realization of a display of a higher resolution.
In an LCD, one switching element (TFT) is used and is directly connected to a pixel electrode. In an organic EL panel, on the other hand, two TFTs and one capacitor are used. FIG. 11 shows an example structure of a pixel circuit in a conventional organic EL panel which uses thin film transistors (TFT). An organic EL panel is constructed by arranging such pixels in a matrix form.
Agate of a first TFT 10 which is an n channel thin film transistor selected by a gate line GL is connected to the gate line GL extending in the row direction. A drain of the first TFT 10 is connected to a data line DL extending in the column direction and a source of the first TFT 10 is connected to a storage capacitor CS whose other terminal is connected to a capacitance line SL which is a low potential power supply. The connection point between the source of the first TFT 10 and the storage capacitor CS is connected to a gate of a second TFT 40 which is a p channel thin film transistor. A source of the second TFT 40 is connected to a power supply line VL and a drain of the second TFT 40 is connected to an organic EL element EL. The other terminal of the organic EL element EL is connected to a cathode power supply CV.
In this structure, when the gate line GL is high (H level), the first TFT 10 is switched on and data on the data line DL is stored in the storage capacitor CS. An electric current in the second TFT 40 is controlled according to the data (potential) maintained in the storage capacitor CS, an electric current flows through the organic EL element EL according to the electric current in the second TFT 40, and light is emitted.
Whenever the first TFT 10 is switched on, a video signal corresponding to the pixel is supplied to the data line DL. The storage capacitor is then charged according to the video signal supplied to the data line DL, the second TFT 40 causes a corresponding electric current to flow, and the brightness of the organic EL element EL is controlled. The electric current flowing through the organic EL element EL is controlled by controlling the gate potential of the second TFT 40 and a gradation display for each pixel can be achieved.
In such an organic EL panel, a defect may occur in the first TFT 10 or the second TFT 40 provided for each pixel. If the defect is such that the electric current to the organic EL element is fixed at the “off” state, the pixel becomes a continuously dark point. Such a dark point is relatively not problematic because it is difficult for an observer to recognize one dark point among surrounding pixels that are normally emitting light. On the other hand, if the defect of the TFT is such that the electric current to the organic EL element is fixed at the “on” state, this pixel becomes a bright point. A bright point is conspicuous, even when only one bright point is present among surrounding pixels displaying black, and the bright point is therefore viewed as a serious deficiency. Therefore, conventionally, processes for darkening the defective pixel which became a bright point are applied.
In other words, an organic EL panel with a predetermined number of dark points is not considered problematic as a product, and the yield can be significantly improved by attenuating light of the bright point.
The darkening process can be performed by, for example, severing the wiring to the pixel. That is, similar to the LCDs, a method can be considered wherein the wiring between the second TFT 40 and the power supply line or between the second TFT 40 and the pixel electrode is cut using a YAG laser or the like.
By such a process, it is possible to address the problem of perceivable display quality by darkening the bright points.
However, when a darkening process using a YAG laser is applied, the cathode is damaged and display of other pixels may be affected. Specifically, in an active matrix organic EL panel, a TFT is formed on a glass substrate, an ITO anode is formed above the TFT, one or more organic layers such as a hole transport layer, an organic emissive layer, and an electron transport layer are layered on the ITO anode, and a metal cathode is formed on the organic layers. As described, a portion of the organic layers and the cathode are present above the TFT. In particular, the cathode is provided as a common electrode and formed over almost the entire surface of the panel.
With such a structure, when the wiring to the TFT is severed using a YAG laser, the laser may reach the cathode, causing ablation in the cathode and creating a hole in the corresponding section of the cathode. In addition, the ablation alters the characteristics of the cathode and the display of the surrounding pixels may also be affected. Furthermore, because the cutting process using a laser is a process in which the material present at the site is evaporated and removed, the side surfaces of the organic layers of the organic EL element become directly exposed to the environment above the cathode. Presence of such an exposed section increases the tendency for degradation of the organic layers by intrusion of moisture or the like, and may lead to formation of additional defective pixels.